The present invention relates to a method and apparatus for stabilizing and brightening a prerecorded TV signal that has been encoded with copy protection pulses, while still preserving those pulses. More particularly, the invention relates to a method and apparatus for providing a clear video image when viewing prerecorded tapes with copy protection by adjusting the relationships between sync pulses, color bursts and video information in a signal that has been degraded by the addition of copy protection pseudo-sync and automatic gain control (AGC) pulses that act to prevent copying of the video signal.
The producers of video tape programming have sought to limit the ability of consumers to make copies of the tapes on conventional video tape recorders ("VCR's"). The thought is that the making of such copies will reduce the market for the original tapes, i.e., through direct purchase or rental. This is not entirely correct since VCR's produce analog copies of the original signal which are always inferior to the original. Also, some copies are made merely for the purpose of legitimate time shifting. For example, if a tape is rented it must be returned in a certain period of time. If the renter did not have a chance to view it by that time, it would presumably be acceptable to make a copy, view it later and then record over it. This time shifting would have no adverse effect on the market for the original.
Early attempts to render tapes difficult to copy involved suppression of the sync pulses. The idea was that the sync pulses would be large enough to allow viewing of the tape, but the degradation in copying the tape would make the sync pulses too small to provide a stable picture when the copy was viewed. However, since VCR's and television ("t.v.") monitors have different sync sensitivities, this copy protection system failed. Sometimes it allowed acceptable copies to be made and sometimes it prevented viewing originals.
An improved system for preventing the copying of video recordings is described in U.S. Pat. No. 4,631,603 of Ryan. This patent relies on the fact that most VCR's have an automatic gain control ("AGC") circuit which sets the level of the recorded signal. This AGC circuit recognizes the difference between the sync tip and the back porch in the video signal, which are typically at IRE levels of -40 and 0, respectively. In the Ryan system, pseudo-sync pulses are added after the normal sync pulses in the twelve lines following the post equalizing period of the vertical blanking interval. These extra sync pulses cause the recording VCR to lose synchronization. Further, each pseudo-sync pulse may be followed by an AGC pulse which may vary between 0 IRE and IRE 100, i.e., 100% white level, and is located in the same relative position to the pseudo-sync as the back porch is to normal sync. These AGC pulses in the original tape act on a t.v. monitor's AGC circuit resulting in a darkened t.v. picture. Also, if this video signal is copied on to a tape in a second VCR, the recording VCR's AGC circuit assumes that the input video is very large and reduces the gain, again resulting in a darkened t.v. picture when the copied tape is played. This signal, however, is darkened more severely than when the original copy protected tape is viewed on a t.v. monitor. In one version the AGC pulses are inserted randomly, so the t.v. picture flashes light and dark. Also, all the sync pulses can be suppressed to add synchronizing instability.
The intent of the Ryan system is to provide copy protection without adversely affecting normal viewing of the original pre-recorded video tape. Unfortunately, the added pseudo-sync and AGC pulses, and the reduced sync pulses do have an adverse effect on the normal viewing of originally produced copy protected tapes, particularly on older television models. The picture may have distortions in lumination and synchronization instability, i.e., jitter or wavy lines. This has been reported in the press. See D. Dupre, Macro-Scrubber Macrovision Stabilizer, RADIO ELECTRONICS, December 1987, at 50.
One solution to the problem caused by these added pulses is to simply remove the unwanted pulses. U.S. Pat. No. 4,870,297 to Karlock, U.S. Pat. No. 4,695,901 to Ryan and U.S. Pat. No. 4,336,554 to Okada disclose methods and apparatus for removing these pulses by blanking the signal in the portion of VBI where they occur. However, there are reasons to retain this VBI interval unchanged, i.e. to maintain the copy protection, and to protect VITS, teletex, GCR and bilingual data that occur in the eleven line VBI interval.